Stall protection based on back EMF detection
A method of detecting a rotor condition of a dc motor is provided. The motor 14 is part of a motor control circuit 10 having a free-wheeling diode 20 in parallel with the motor 14. The method establishes a threshold voltage for back emf voltage evaluation. The motor is energized to operate at a certain speed. Current is cut-off to the motor via a switch 16 for a time sufficient for current through the free-wheeling diode to decay to zero. The back emf voltage detected at leads of the motor is then compared via a comparator 21 to the threshold voltage.
This application is based on and claims priority from U.S. Provisional patent Application No. 60/379,318, filed May 9, 2002, the contents of which is hereby incorporated into the present specification by this reference.
FIELD OF THE INVENTIONThe invention relates to detecting a blocked rotor condition of a dc motor and more particularly, detecting a blocked rotor condition at motor start up and during motor operation.
BACKGROUND OF THE INVENTIONDirect current motors are commonly used in vehicle engine cooling systems to drive, for example, and engine cooling fan. In the event a dc motor is stalled, for example if some foreign object is blocking the fan wheel or the motor, some form of blocked rotor protection is required from damaging the motor or wiring harness. Without stall protection, it is possible for electrical damage or even fire to occur from overheating of the motor.
Accordingly, there is a need to provide blocked rotor or stall protection both at motor start up and during motor operation no matter how the motor is operating (e.g., PWM mode or dc mode).
SUMMARY OF THE INVENTIONAn object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing a motor control circuit for detecting a condition for a rotor of a dc motor. The motor control circuit includes a dc motor having a rotor. A power supply supplies power to the motor. A free-wheeling diode is in parallel with the motor. A switch is provided to cut-off current to the motor. At least one diode is connected in series with a positive terminal of the motor to establish a voltage threshold. An open collector comparator is provided and a voltage tap is provided to detect voltage across leads of the motor. A controller is constructed and arranged to control operation of the switch such that when the switch is turned off, cutting-off current to the motor, back emf voltage can be detected via the voltage tap and be compared via the comparator to the threshold voltage.
In accordance with another aspect of the invention a method of detecting a rotor condition of a dc motor is provided. The motor is part of a motor control circuit having a free-wheeling diode in parallel with the motor. The method establishes a threshold voltage for back emf voltage evaluation. The motor is energized to operate at a certain speed. Current is cut-off to the motor via a switch for a time sufficient for current through the free-wheeling diode to decay to zero. The back emf voltage detected at leads of the motor is then compared via a comparator to the threshold voltage.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.
The invention will be better understood from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which:
With reference to
In
For pulse width modulation (PWM) control of the motor 14, the switch 16 is preferably a FET or MOSFET that is controlled by a PWM signal received from a switch controller 18. However, in the broadest aspect of the invention, the switch 16 can be a relay or any device capable of cutting-off current to the motor 14.
The operation of the circuit 10 is as follows. The back emf voltage generated by the motor 14 can be measured at the motor leads, via a voltage tap 25, (
With reference to
(In+)=[(M+)−2*Vf]*k
(In−)=(M−)*k
(In+)−(In−)=[(M+)−(M−)−2*Vf]*k
where:
-
- In+ is the non-inverting input of the comparator
- In− is the inverting input of the comparator
- M+ is the positive terminal of the motor
- M− is the negative terminal of the motor
- Vf is the forward voltage across a diode (e.g., diode 22)
- k is a resistor divider ratio, such as:
k=R2/(R1+R2)=R4/(R3+R4)
In the above formula, [(M+)−(M−)] is the back emf voltage. A stall condition is indicated when In−(M−) is greater than In+(M+). The motor 14 is considered to be running properly when In+(M+) is greater than In−(M−). The motor 14 is considered to be blocked when (M+) and (M−) are at the same potential.
The method described herein can accommodate the detection of a partial stall condition of the rotor of a motor. As used herein “partial stall” means that the rotor is not in a fully blocked condition. To detect a partial stall condition, the fixed voltage threshold is replaced with a voltage proportional to the expected motor speed. For example, under PWM control a motor would operate at a certain speed with a back emf voltage expected to be, for example, 6 volts. If the threshold voltage is set at 5.5 volts for this speed, and the back emf voltage is detected to be 5 volts, this indicates that the motor is running slow. When a blocked rotor or a partial stall condition of the rotor is detected, a conventional protection scheme can be employed to protect the motor 14 and/or circuit 10 from damage.
Another function of the circuit 10 is to perform the first measurement of the back emf voltage after a time long enough to allow the motor to build up speed. A timing compromise must be achieved in order to fulfill the following two goals:
-
- First back emf check to be performed after a time long enough so that the motor can build up speed (e.g., back emf), and
- Periodic back emf check, when the motor is running, to be performed often enough so that in case of a blocked rotor condition, the electrical stress (over-current) applied to the motor and electronics is minimized.
The detection of rotor condition (e.g., blocked rotor) of the dc motor 14 is independent of the driving circuit such as single or variable speed, battery voltage, and motor lead resistance. If PWM control is used, the switching frequency, PWM duty cycle and low side or high side switching element, are transparent to the blocked rotor detection method as described above.
The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.
Claims
1. Method of detecting a rotor condition of a dc motor, the motor being part of a motor control circuit, the motor control circuit having a free-wheeling diode in parallel with the motor, the method including:
- establishing a threshold voltage for back emf evaluation by providing at least one diode connected in series with a positive motor terminal,
- energizing the motor to operate at a certain speed,
- cutting-off current to the motor for a time sufficient for current through the free-wheeling diode to decay to zero, and
- comparing the back emf voltage detected at leads of the motor to the threshold voltage.
2. The method of claim 1, wherein the step of cutting-off current to the motor includes controlling a switch to be turned-off preventing current to the motor.
3. The method of claim 2, wherein switch is a FET controlled by a pulse width modulated signal, and the method includes controlling the FET to cut-off current to the motor.
4. The method of claim 3, wherein the sufficient time does not exceed one pulse duration.
5. The method of claim 1, wherein the step of comparing the back emf includes using an open collector comparator and, the following formulas being used in the back emf voltage and threshold voltage comparison: where:
- (In+)=[(M+)−2*Vf]*k
- (In−)=(M−)*k
- (In+)−(In−)=[(M+)−(M−)−2*Vf]*k
- In+ is the non-inverting input of the comparator
- In− is the inverting input of the comparator
- M+ is the positive terminal of the motor
- M− is the negative terminal of the motor
- Vf is the forward voltage across the at least one diode
- k is a resistor divider ratio of values of resistors in series with the comparator,
- wherein a stall condition is indicated when In−(M−) is greater than In+(M+) and the motor is considered to be running properly when In+(M+) is greater than In−(M−).
6. The method of claim 1, wherein the energizing step includes operating the motor in a DC control mode, and the cutting-off step includes controlling a switch to be off only when a stall condition of the motor is to be determined.
7. The method of claim 1, wherein the step of establishing a threshold voltage includes establishing a fixed threshold voltage.
8. The method of claim 1, wherein the step of defining a threshold voltage includes establishing a threshold voltage proportional to an expected speed of the motor.
9. A motor control circuit for detecting a condition of a rotor of a motor of the circuit, the circuit comprising:
- a dc motor having a rotor,
- a free-wheeling diode in parallel with the motor,
- diode means for establishing a threshold voltage for back emf evaluation, the diode means including at least one diode connected in series with a positive motor terminal,
- means for energizing the motor to operate at a certain speed,
- means for cutting-off current to the motor for a time sufficient for current through the free-wheeling diode to decay to zero, and
- means for comparing the back emf voltage at leads of the motor to the threshold voltage.
10. The circuit of claim 9, wherein the means for cutting-off current to the motor is a switch.
11. The circuit of claim 10, wherein switch is a FET controlled by a pulse width modulated signal.
12. The circuit of claim 11, wherein the sufficient time does not exceed one pulse duration.
13. The circuit of claim 9, wherein the means for comparing includes an open collector comparator, the following formulas being employed in the back emf voltage and threshold voltage comparison: where:
- (In+)=[(M+)−2*Vf]*k
- (In−)=(M−)*k
- (In+)−(In−)=[(M+)−(M−)−2*Vf]*k
- In+ is the non-inverting input of the comparator
- In− is the inverting input of the comparator
- M+ is the positive terminal of the motor
- M− is the negative terminal of the motor
- Vf is the forward voltage across the free-wheeling diode
- k is a resistor divider ratio of values of resistors in series with the comparator,
- wherein a stall condition is indicated when In−(M−) is greater than In+(M+) and the motor is considered to be running properly when In+(M+) is greater than In−(M−).
14. A motor control circuit for detecting a condition for a rotor of a dc motor, the motor control circuit comprising:
- a dc motor having a rotor,
- a power supply supplying power to the motor,
- a free-wheeling diode in parallel with the motor,
- a switch for cutting-off current to the motor,
- at least one diode, connected in series with a positive terminal of the motor for establishing a voltage threshold,
- an open collector comparator,
- a voltage tap for detecting voltage across leads of the motor, and
- a controller constructed and arranged to control operation of the switch such that when the switch is turned off, cutting-off current to the motor, back emf voltage can be measured via the voltage tap and can be compared via the comparator to the threshold voltage.
15. The circuit of claim 14, wherein the switch is a FET.
16. The circuit of claim 14, wherein the switch is a relay.
17. The method of claim 1, wherein the step of comparing the back emf includes using an open collector comparator.
18. The circuit of claim 9, wherein the means for comparing the back emf is an open collector comparator.
19. The circuit of claim 14, wherein the comparator employs the following formulas in the back emf voltage and threshold voltage comparison: where:
- (In+)=[(M+)−2*Vf]*k
- (In−)=(M−)*k
- (In+)−(In−)=[(M+)−(M−)−2*Vf]*k
- In+ is the non-inverting input of the comparator
- In− is the inverting input of the comparator
- M+ is the positive terminal of the motor
- M− is the negative terminal of the motor
- Vf is the forward voltage across the free-wheeling diode
- k is a resistor divider ratio of values of resistors in series with the comparator,
- wherein a stall condition is indicated when In−(M−) is greater than In+(M+) and the motor is considered to be running properly when In+(M+) is greater than In−(M−).
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Type: Grant
Filed: May 28, 2002
Date of Patent: Jan 4, 2005
Patent Publication Number: 20030210011
Assignee: Siemens VDO Automotive Inc. (Mississauga)
Inventors: Corneliu Dragoi (Ontario), Rodney Vanwyck (Ontario)
Primary Examiner: David Martin
Assistant Examiner: Tyrone Smith
Application Number: 10/157,378